The prediction of ship motion characteristics early in the design stage in realistic sea conditions are of vital importance for the ship designer. Strip theories are commonly used for this purpose as they are fast and inexpensive. In this paper, an existing two-dimensional time-domain strip theory optimised for multi-hull vessels travelling at high Froude numbers is extended to predict motions in an irregular seaway. The encountered wave environment is represented by the superposition of regular sinusoidal waves. A method for decomposing idealised sea spectra into component regular waves of varying frequencies and constant amplitudes is presented. Ship motion predictions in irregular waves are then verified by conducting a spectral analysis on the motions and wave environment and comparing with motion predictions in regular waves. A method of ensemble averaging of spectra over a series of runs is adopted to reduce spectral variance. The extended seakeeping method is then validated by comparing predicted motions of a large high-speed catamaran in irregular seas with scale model results from towing tank experiments.
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